1. Field of the Invention
Aspects of the present invention relate to a hologram recording and/or reproducing apparatus, a hologram recording and/or reproducing method, and a hologram recording medium, and more particularly, to a hologram recording and/or reproducing apparatus, a hologram recording and/or reproducing method, and a hologram recording medium which enhance compatibility between apparatuses for recording and/or reproducing holograms.
2. Description of the Related Art
Since an optical system using a lens is used to reproduce data on a hologram recording medium, aberrations among reproducing apparatuses are not identical. The aberrations are primarily generated by the lens, but the aberrations may also be generated by other optical components as well, such as prisms, mirrors, and light sources. When data is recorded on a hologram recording medium by one apparatus and reproduced by another apparatus, and the two apparatuses have different aberrations, the quality of the reproduced signal is degraded. Since hologram recording requires the use of a high-density method requiring up to 1 terabyte capacities of memory, degradation of the signal by aberrations is significant.
FIG. 1 illustrates a reference page used for position correction and a data page according to conventional technology. As illustrated in FIG. 1, a reference page 101 is recorded first. Then, a data page is recorded. The reference page 101 is used to correct signal degradation caused by shifting, expansion, or reduction of the entire image when data is recorded and reproduced, and includes markers functioning as references for position correction. When data is reproduced, the reference page 101 is reproduced first, and reference markers of the data page 102 are set based on the reference markers positioned at the four corners of the reference page 101. Then, by matching the positions of the reference markers at the four corners on the data page 102 with the positions of the reference markers of the reference page 101, the position of the data page 102 is corrected to correct a distortion of the entire image which occurs during data reproduction.
FIG. 2 illustrates the shape of a page 200 generated from the reference page 101 and the data page 102 of FIG. 1. As illustrated in FIG. 2, by recording and reproducing the page 200 so that the reference position markers 202 and data 201 are formed together, the positions may also be corrected.
However, when a recorded image experiences partial deformation caused by an aberration of an optical apparatus, the signal quality cannot be corrected simply by correcting shifting, expansion or reduction of the entire image. For instance, if a spherical aberration causes blurring in the center of the image, then even though there is no deformation at the edges of the image, degradation of a signal occurring by deformation, which is caused by distortion of images with respect to positions of a data page, occurs. When data recorded on a hologram recording medium is reproduced using the same apparatus that recorded the data, the error is predictable because the degradation component caused by the aberration is identical both during the recording of and reproducing of the data. Accordingly, when the same apparatus is used to record and reproduce data recorded on a hologram recording medium, any degradation which occurs may be corrected by image processing.
However, when the apparatus used to reproduce data is different from the apparatus used to record the data, the optical system used to reproduce the data is different from the optical system used to record the data, and thus, the aberration changes. Accordingly, when hologram recording and reproducing apparatuses having different optical system characteristics are used, the image is degraded by the change in the aberration, and the reproduced signal deteriorates further.
FIG. 3 illustrates a method of recording a hologram according to conventional technology. In a hologram recording and reproducing system, a laser beam having an interference characteristic is split into a signal beam and a reference beam, and the intensity of the signal beam is modulated with respect to the data to be recorded by a spatial light modulator (SLM) 301. The modulated signal beam and the reference beam are then combined on a hologram recording medium 302. The signal beam and the reference beam interfere with each other and the resulting interference pattern is recorded as a recording spot (#1, #2, #3, . . . , #n) on the recording medium 302. The recording spots are then recorded at different positions as the hologram recording medium 302 is rotated in the direction indicated by an arrow.
In order to reproduce data recorded on a hologram recording medium, an illumination beam, having the same characteristics as the reference beam, is applied to the hologram recording medium at the same angle as the reference beam. Then, data is reproduced as a diffraction beam corresponding to the interference pattern recorded on the hologram recording medium. This diffraction beam is then collected by an image pickup device, such as a CCD or a CMOS device, and received as a bit pattern. The light signal received by the image pickup device is interpreted and reproduced as data.
FIG. 4 illustrates degradation of a signal due to an optical path difference (OPD). As illustrated in FIG. 4, degradation of the signal caused by defocusing aberration, which in turn is caused by the OPD, is relatively small, while the influence of degradation caused by spherical aberration, distortion and coma is relatively large. Generally, a total of 0.05 λrms of the OPD of an optical system may occur in a recording or reproducing apparatus. Thus, the maximum OPD which may occur when using an apparatus to record data which is different from an apparatus used to reproduce data is the sum of the recording and reproducing aberrations (0.05 λrms +0.05 λrms), or approximately 0.1 λrms. Referring to FIG. 4, it can be seen that the degradation of the signal increases up to at least several decibels.
Meanwhile, a laser diode currently sold by Sony to be used with holographic memory has an aberration higher than 0.05 λrms. If the influence of this aberration is also considered, the maximum aberration that can occur between hologram recording and reproducing apparatuses can reach 0.2 λrms, causing signal degradation of up to 60%, as illustrated in FIG. 4. This level of degradation may vary depending on which noise components are included in the system and the signal.